Correction: The earliest source for saying hatoras nedarim with 10 people is the Pri Aitz Chaim (Shaar Rosh HaShannah). The reason is explained (elsewhere I believe) as follows: since we say in hatoras nedarim, “she’nadarti oi she’nishbati BA’CHALOM” and in the Shulchan Aruch (YD 210:2) it says that one should be concerned (or there are those who are of the opinion one should be concerned) for those who say one who makes a neder in a chalom requires 10 to anul the neder, it is proper to gather 10 men for this purpose. But I believe the prevailing minhag appears to be as I wrote above.